The present invention relates to an improved method and an improved plant for treating milk so as to obtain milk with a reduced content of spores and bacteria, wherein low-fat milk, such as skim milk, is subjected to microfiltration causing a separation into a spore- and bacteria-containing retentate and a permeate in form of a milk fraction, the content of milk proteins being substantially maintained and the content of spores and bacteria being considerably reduced. The treated low-fat milk may be used in this form or may be mixed with a fatty milk fraction, such as cream, which has been subjected to a bacteria-controlling treatment, eg a heat treatment, so as to produce standardized milk. The treated milk and the standardized milk are both suitable for direct consumption and as raw material for processed dairy products, eg for making cheese. The improvement in the method and the plant is in a special arrangement of the equipment for the membrane filtration which renders a more efficient removal of bacteria and spores and which is more safe in case of a membrane breakdown.
Danish printed accepted application No. 164.722 and the corresponding EP patent No. 0 194 286 (Holm et al.) disclose a plant for treating milk in such a manner that the milk has a low bacterial content. Fatty milk is divided by centrifugation into a cream fraction and a skim milk fraction. The skim milk fraction is caused to pass through a microfilter, in which the fat globules and the bacteria are separated off. The microfiltration results in a permeate consisting of skim milk with a low bacterial content and a retentate (concentrate) having a higher content of fat and bacteria than the permeate. The retentate is combined with the cream fraction resulting from the centrifugation, and the obtained mixture is sterilised. The sterilised material or a portion thereof is combined with the permeate to obtain milk with the desired fat content. The advantage of this known method is that only a minor fraction of the milk need be sterilised in order nevertheless to obtain standardised milk with a low bacterial content. The combination of a centrifuigal separation and microfiltration provides a significantly increased capacity of the microfilter.
DK 169 510 and the corresponding EP 0 697 816 (Krabsen et al.) disclose a similar plant, in which, however the retentate resulting from the microfiltration is recirculated to centrifugal separator, ie mixed with the added milk and centrifuged therewith, instead of being combined with the cream fraction. Bacteria and spores thus being recirculated to the centrifugal separator, are, however, not accumulated in the plant, as they are continuously or discontinuously removed with a sludge fraction. This possibility of removing sludge is known from many conventional centrifugal separators.
Microfiltration processes using the cross-flow principle, eg the processes used in the above plants, may be carried out by employing conventional microfiltration units of differing structural shapes. As a basic model a microfiltration unit (MF unit) with cross flow may be formed of a container divided by a microfiltration membrane into two chambers, a feed/rententate chamber and a permeate chamber. The retentate chamber is provided with a feed conduit for feeding the material to be filtered, and a retentate outlet. The permeate chamber is provided with a permeate outlet. Between the retentate chamber and the permeate chamber a pressure difference is etablished forcing the fluid and small particles through the membrane. The feed material is fed through the retentate chamber from one side along the membrane. On the other side of the retentate chamber the retentate is removed, said retentate consisting of the fluid and the particles, which have not passed through the membrane to the permeate chamber during the passage along the membrane. In order to prevent the membrane surface from being fouled too quickly, which causes clogging of the membrane pores, the flow rate (cross-flow rate) over the surface of the membrane should not be too low. This is often ensured by recirculating a portion of the retentate flow to the feed conduit. It is also well-known to recirculate a portion of the permeate to ensure a uniform pressure drop, the permeate chamber in addition to the permeate outlet also being provided with an inlet for receiving recirculated permeate. This principle is described in U.S. Pat. No. 4,105,547 (Sandblom). Such recirculation conduits for retentate or permeate leading to the same respective retentate chamber or permeate chamber from which said material has flown, are considered as components forming part of a basic model of the microfiltration unit.
For large scale plants a larger membrane area may be needed and often this is obtained by interconnecting a large or small number of the above basic models. Accordingly, a large filtration area may be obtained by parallel coupling several basic models. This principle is for instance described in connection with ultrafiltration of whey by Rud Frik Madsen in xe2x80x9cHyperfiltration and Ultrafiltration in Plate-and-Frame Systemsxe2x80x9d, Elsevier, 1977, page 134, FIG. 4.23. It is also known to interconnect several filtration units in series such that the portion of the retentate resulting from the first unit, which is not recirculated, is added as feed material to the subsequent filtration unit, etc. This principle is for instance shown in Perry""s Chemical Engineers"" Handbook, 6th edition 1984, page 17-32, FIG. 17-29.
U.S. Pat. No. 5,685,990 (Saugmann et al.) discloses how to membrane filtrate an aqueous dispersion by employing several primary membrane units interconnected in such a manner that the retentate or a portion of the retentate resulting from a membrane filtration step is used as feed material for one or more subsequent steps, while the permeate from said primary filtration steps is concentrated by evaporation or in a secondary membrane filtration step, in which the concentrate or the secondary retentate is recirculated to the aqueous feed dispersion in one or more of the primary filtration steps. As an essential feature the membranes in the secondary membrane filtration step should have a smaller pore size or molecular cut-off value in relation to the membranes in the primary filtration steps. Examples of the primary filters are ultrafiltration filters (UF filters), while the secondary filters may be hyperfiltration filters (HF filters), which are also known as RO filter, RO denoting reversed osmosis.
WO 94/13148 (Bounous et al.) discloses a process for producing an undenatured whey protein concentrate from skim milk, microfiltration being carried out in a first step with a microfilter retaining bacteria, but allowing the skim milk containing both whey proteins and other milk proteins, such as casein, to pass through the filter, and in a subsequent step microfiltration being carried out with another type of microfilter retaining casein, but allowing the whey proteins to pass. The known method thus cannot be used for producing a milk fraction in which the content of all types of milk proteins, ie both casein and whey proteins, are substantially maintained, while the content of spores and bacteria is considerably reduced.
The use of microfiltration for removing bacteria from a low-fat milk fraction as described in DK 164.722 and DK 169.510 is advantageous in that the bacteria may be removed without heat treatment which is substantially more gentle to the milk components. As a result the good taste is preserved and a denaturation of proteins and other changes of the properties of the milk can be avoided. In addition it is prevented that the milk fraction contains heat-treated and thus dead bacteria. Even when the skim milk fraction subsequently is to obtain a desired fat content by being mixed with heat-treated cream, the result is still an improved product as regards taste and the preservation of proteins. Products treated in this manner are suitable both for direct consumption and as raw material for processed milk products, such as yoghurt and cheese.
Today""s microfiltration membranes are highly reliable and membrane breakdowns are very rare. Although the probability of a membrane breakdown is very small, the risk cannot be completely excluded. Membrane breakdowns do cause serious problems. It is a problemxe2x80x94even at frequent sampling for determining bacteriaxe2x80x94that the result of such a determination usually is not available until one or several days after the sampling. Thus, several days may pass before it can be ascertained whether the permeate from the microfiltration has a too high bacterial count. When producing non-industrial milk, the milk is usually tapped shortly after the microfiltration for which reason the risk exists that large amounts of milk either have to be discarded or used to another purpose. However the tapped milk may already have been distributed to the stores and further on to the consumers before the high bacterial count has been found, which may entail a withdrawal of the milk and pose a health risk to the consumers. Such incidences may be highly detrimental to the goodwill and economy of the dairy.
The above serious problems in connection with membrane breakdowns entail that the authorities as a rule demand that microfiltrated milk for consumption must be subjected to a minimum of heat treatment, eg pasteurization at 72xc2x0 C. for fifteen seconds as a supplement to the microfiltration. This is primarily demanded in order to eliminate the presence of pathogenic bacteria.
The supplementary heat treatment limits the above advantages of the use of microfiltration instead of the conventional bacterial destruction by heat treatment. Also at this mild pasteurization undesirable changes occur in the properties of the milkxe2x80x94although on comparatively small scale.
A need thus exists for carrying out microfiltration of milk in a more secure manner, whereby the damages arising at a membrane breakdown, are practically completely avoided and preferably in such a manner that the supplemental pasteurization is rendered superfluous.
It has now been found that the desired increased safety can be obtained by using at least two membrane units interconnected in a special manner.
The present invention thus relates to a plant for treating low-fat milk, such as skim milk, so as to obtain a reduced content of spores and bacteria and a substantially unchanged content of milk proteins, said plant comprising a feed conduit for milk coupled to a first microfiltration unit, MF-I, separating the milk into a spore- and bacteria-containing first retentate, R-I, and a first permeate, P-I, with a lower content of spores and bacteria, said microfiltration unit, MF-I, being coupled to a conduit for the first retentate, R-I, and a conduit for the first permeate, P-I. The plant is characterised in that after the microfiltration unit, MF-I, the permeate conduit is coupled to a second microfiltration unit, MF-II, for separating the first permeate, P-I, into a second retentate, R-II, and a second permeate, P-II, said second microfiltration unit, MF-II, being coupled to a conduit for the second retentate, R-II, in form of a recirculation conduit leading to the first microfiltration unit, MF-I, and to a conduit for the second permeate, P-II.
The invention also relates to a method of treating low-fat milk, such as skim milk, so as to obtain milk with a reduced content of spores and bacteria and a substantially unchanged content of milk proteins, where the milk is subjected to microfiltration causing a separation into a spore- and bacteria-containing retentate and a permeate with a reduced content of spores and bacteria, said method being characterised in that the permeate resulting from the microfiltration is subjected to an additional microfiltration.
According to a preferred embodiment of the method according to the invention the retentate from the second microfiltration step is recirculated to the feed side for the first microfiltration step.
The special interconnection of two or optionally more microfiltration units increases the safety significantly, the product flow, ie the second permeate, having passed through two independent microfiltration membranes. As the probability of a microfiltration membrane breakdown is very small, as mentioned above, simultaneous breakdowns of both microfiltration membranes are highly unlikely.
Conventional filtration processes can be performed as so-called dead-end-filtrations, in which a fluid containing a sediment is led through a filter retaining the sediment and allowing the filtrate to pass through the filter. The drawback of dead-end-filtration is that the openings of the filter clog quickly, whereby the flow through the filter, the flux, quickly drops to an unacceptable low level. Consequently dead-end-filtration has primarily been used for macrofiltration, where the openings in the filter exceeds 5-10 xcexcm and where the sediment consists of comparativel large particles such that the filter cake has suitable openings to allow the filtrate to pass therethrough.
At membrane filtration including microfiltration, where the openings or pores for passage through the membrane are less than 2-5 xcexcm and where the particles or molecules to be retained as xe2x80x9csedimentxe2x80x9d, typically are of a size only slightly larger than the pores, dead-end-filtration usually cannot be used, as the filter clogs all too quickly. The above cross-flow principle solves this problem, as the sediment is removed as a flowable phase, the retentate, which in theory may be regarded as a dispersion of the sediment in a dispersion medium. In this case the dispersion medium has substantially the same composition as the permeate.
The advantage of cross flow is in the increased flux which is a condition for the operating economy. A drawback is that a portion of the material intended to be transferred to the permeate remains in the retentate. In such cases, where the permeate contains the desired end product, the retentate is conventionally subjected to an additional separation treatment, eg an additional membrane filtration. The conventional series coupling of several membrane units thus is based on the principle for instance described in Perry""s Chemical Engineers"" Handbook, 6th edition 1984, page 17-32, FIG. 17-29, in which the retentate from the first membrane filtration unit is led to the subsequent unit. The present invention departs from this conventional principle in that in this case it is the permeate and not the retentate from the first unit which is subjected to an additional microfiltration.
When microfiltrating low-fat milk for removal of bacteria and bacterial spores by employing the cross-flow principle, a portion of the milk protein and other milk components, which would have been valuable components of the product, ie ideally transferred to the permeate, flow out with the retentate. Moreover also a small quantity of spores and bacteria, which should have been retained by the filter, pass through the membrane to the permeate due to variations in the pore sizes of the membranes, whereby a small number pores may be present of a sufficiently large size to allow some bacteria and in particular bacteria spores to pass therethrough.
By using the special coupling of MF units according to the invention, in case of a breakdown of one of MF units, the second permeate has a slightly higher bacterial count which is detected at the regular bacteriological control tests. However, the increase in the bacterial count is only relatively modest such that the product tapped during the period of time from the membrane breakdown to the result of the control test is known, still has a bacterial count which is within the acceptable limits.
The above advantage is not obtained when using the conventional membrane unit couplings. In the conventional parallel coupling merely an increased membrane area is obtained, an assembled system of membranes coupled in parallel in reality operating as a single membrane unit having a large surface.
In the conventional series coupling described in Perry""s Chemical Engineers"" Handbook, 6th edition 1984, page 17-32, FIG. 17-29, in which the retentate from the first membrane unit is led as feed material to the subsequent membrane,unit, a breakdown of merely one of the membrane units allow bacteria direct access to the product flow. The use of several membrane units thus does not increase the safety.
In case low-fat milk is microfiltered according to the principle used according to U.S. Pat. No. 5,685,990 (Saugmann et al.), ie, the first filtration unit having a considerably larger pore size than the second filtration unit, a comparatively large number of bacteria and bacteria spores pass through the first filtration unit, which thus only acts as a preliminary coarse filtration, whereafter the spores and bacteria are retained by the second filtration unit. In this case the prevention of bacteria and spores passing into the treated milkxe2x80x94in particular at a breakdown of the second filtration unitxe2x80x94is considerably smaller compared to the risk with two intercoupled microfiltration units, which both efficiently retain bacteria, according to the idea of the present invention.
The extent of applicability of the invention appears from the following detailed description. It should, however, be understood that the detailed description and the specific examples are merely included to illustrate the preferred embodiments, and that various alterations and modifications within the scope of protection will be obvious to persons skilled in the art on the basis of the detailed description.
The plant according to the invention is suitable for the removal of bacteria from low-fat milk, such as skim milk, prepared at conventional centrifugation. The plant may advantageously be combined with a centrifugal separator and may thus comprise a centrifugal separator unit for separating the milk into a cream fraction CR, a skim milk fraction SM and optionally a sludge fraction SL; a conduit for the skim milk fraction coupled to a first microfiltration unit MF-I for separating the skim milk fraction SM into a spore- and bacteria-containing first retentate R-I and a first permeate P-I with a lower content of spores and bacteria, MF-I being coupled to a conduit for R-I and a conduit for P-I and the conduit for P-I being coupled to a second microfiltration unit MF-II for separating the first permeate P-I into a second retentate R-II and a second permeate P-II, MF-II being coupled to a conduit for R-II optionally in form of a recirculation conduit leading to the first microfiltration unit MF-I, and to a conduit for P-II, respectively.
In such a plant the centrifugal separator unit is further coupled to a conduit for the cream fraction CR. The cream conduit may in turn be coupled to a bacteria-controlling unit which in turn is coupled to a conduit for the cream treated in the bacteria-controlling unit, said conduit being coupled to a uniting conduit to which the conduit for the second permeate P-II is coupled, in such a manner that the treated cream or a portion thereof and the permeate are united in the uniting conduit so as to form standardized milk.
The term standardized milk denotes a milk product, which by mixing a low-fat milk fraction and the necessary amount of a milk fraction with a high fatty content, such as cream, has been adjusted to a desired fat content standardized for the product type in question.
In order to ensure that bacteria and bacteria spores are retained equally efficiently in both MF units such that only few bacteria pass through to the permeate in case of a membrane breakdownxe2x80x94regardless of which MF unit has broken downxe2x80x94it is preferred that the first microfiltration unit and the second microfiltration unit have substantially the same pore size.
Thus, the average pore sizes of the two membrane units differ less than 50%, preferably less than 20% and most preferably less than 10%, from each other.
The microfiltration units forming part of the plant according to the invention may have any conventional shape. Examples hereof are any type selected from among the plate-and-frame system, a tubular system, a spiral-wound system, a cassette system and the hollow fibre principle or a combination thereof;
Each microfiltration unit may comprise one or more microfiltration membranes preferably having a pore size ranging from 0.1 to 2.0 xcexcm, particularly preferable from 0.4 to 1.8 xcexcm and most preferred from 0.8 to 1.4 xcexcm.
It is essential that both MF units are able to retain bacteria efficiently. If slightly differing pore sizes are used, it is thus essential that the upper limit of about 2.0 xcexcm in pore size is maintained. According to an embodiment the first microfiltration unit may have a pore size ranging from 0.8 to 2.0 xcexcm, while the second microfiltration unit has a pore size ranging from 0.1 to 2.0 xcexcm.
Each MT unit is usually adapted such that the filtration factor calculated as weight amount of retentate in relation to the feed amount ranges from 1 to 20% by weight for each microfiltration step.
By using the plant according to the invention the advantages according to DK 164 722 or DK 169 510 may also be obtained. The conduit for the first retentate (R-I) may thus be coupled such that the first retentate (R-I) is mixed with the cream fraction (CR) prior to the bacteria-controlling unit. Alternatively the conduit for the first retentate (R-I) may be in form of a recirculation conduit leading to the centrifugal separator unit. In the latter case also the conduit for the second retentate (R-II) may be coupled to a feed conduit of the centrifugal separator unit such that both R-I and R-II are recirculated to the centrifugal separator unit as taught in DK 169 510.
In order to further increase the safety the plant according to the invention may comprise three or more membrane units coupled in series, in which the permeate conduit from the second membrane unit (MF-II) is coupled to a third membrane filtration unit (MF-III), the permeate conduit thereof optionally being coupled to one or several subsequent membrane filtration units (MF-IV . . . ),and in which each permeate conduit apart from the last one is coupled to the subsequent membrane filtration unit as its feed conduit and in which at least one retentate conduit is a recirculation conduit for feeding to a preceding membrane filtration unit. By using such an embodiment comprising more than two MF units, the safety is further enhanced. The number of MF units is naturally selected with a view to the increased initial expenditure and the operational expenditure incurring in connection with an increased number of MF units. However to compensate for these increased costs it is possible to form and adjust the individual, especially the latest, MF units for a higher flux and thus to a higher capacity, eg with a slightly larger pore size and/or an increased transmembrane pressure, where the retainment of bacteria may be slightly less efficient for the individual MF unit. In this case the increased number of MF units still efficiently reduces the bacterial count in the last permeate. When three membrane units are used, at least two intact MF units remain in case of a membrane breakdown and it is thus unlikely that the acceptable limits for the bacterial count in the last permeate are exceeded.
In the present description and claims the term microfiltration unit (MF unit) not only denotes the above basic model, but also more complex filtration plants which in principle may be described as several MF basic models parallel coupled in a known manner, in series or by a combination of series and parallel couplings, provided such a MF unit (when seen from the outside) is provided with feeder conduit, a retentate draining conduit and a permeate draining conduit. In this context any recirculation conduits, which do not lead the material away from such a simple or more complex MF unit, are to be regarded as a component of the MF unit in question.